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ll birders know that birds collide with man-made structures. Windows,

fences, transmission lines, motor vehicles, communication towers,

bridges, and wind turbines all kill birds. The number of birds killed by

each of these types of structures varies greatly, but robust, quantitative estimates

of how many birds are killed by most of these structures are not available. Fa-

tality estimates for each of these types of structures range from 100,000 per year

for wind turbines to perhaps 900 million for windows and buildings. These es-

timates, for the most part, are not based on empirical studies, or if they are, the

studies are limited by small sample sizes and non-quantitative methods, and

most are biased geographically.

Communication towers and wind turbines have been the focus of much dis-cussion and concern among birders, environmentalists, and wildlife agencystaffers, mostly because of the proliferation of these structures within the past20 years. In this article, we review what is known about bird fatalities at com-munication towers and wind turbines. Our review provides an introduction to

Paul KerlingerCape May Point, New Jersey

pkerlinger@comcast.net

Joelle GehringLansing, Michigan

gehringj@michigan.gov

Richard CurryMcLean, Virginia

rca1817@aol.com

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the basic structure of communication towers and wind tur-bines, the numbers of these structures in the U.S., the num-bers and types of birds they kill, and a short overview ofongoing studies of impacts caused by these structures. Fi-nally, we offer a perspective on the future of impacts andwhat can be done or is being done to reduce fatalities andprevent significant impacts.

Communication TowersCommunication towers were first reported to kill birds inthe 1940s (Avery 1980). From the late 1950s through the1970s, birders and researchers conducted dozens of infor-mal fatality studies at communication towers in the U.S.Some studies even examined changing numbers of fatalitiesat given towers over several decades (Crawford and En-gstrom 2001, Morris et al. 2003). However, despite the re-cent proliferation of communication towers, there has beenalmost no systematic effort by birders, agencies, and envi-

ronmentalists to understand the variety of towers that killbirds, their support systems, or how they are lit. This infor-mation is critical for designing research and quantitativelydetermining impacts to birds at these structures.

Communication towers are used to provide a platformfor antennae that transmit and/or receive information inthe form of radio waves or microwaves. Tower types in-clude those used for mobile (cellular) telephones, FM andAM radio, television, public safety systems (for example,state police and homeland security), and corporate com-munications. Communication towers range in height fromonly a few feet to slightly more than 2,000 feet. Most tow-ers in excess of 200 feet have at least three sets of guy wires,extending outward to about three-quarters of the towerheight and have some sort of Federal Aviation Administra-tion (FAA) obstruction lighting. These lights may be oneor more of the following: red flashing (incandescent,strobe, or LED), white strobes, and red steady burning(non-flashing). There are more than 100,000 of these com-munication structures registered with the FAA and FederalCommunications Commission (FCC). Their locations aremapped online <towerkill.com>.

Hundreds of millions of passerines migrate acrossNorth America every year. All across their migrationroutes, these birds are confronted with innumerablehuman-made obstacles: buildings, wind turbines,communications towers, and so forth.

American Redstart. Cape May, New Jersey; August 2010.Photo by © Michael O’Brien.

Blackpoll Warbler. Cape May, New Jersey; September 2010.Photo by © Michael O’Brien.

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The American Bird Conservancy (Shire et al. 2000) re-viewed 47 tower studies, concluding that nocturnal mi-grants, mostly songbirds, are killed more than any otherbirds. These towers were all guyed, most were in excess of500 feet, and all were equipped with standard red flashingand red steady burning lights. Of the studies summarized,few were done in a systematic or rigorous fashion and mostfocused on a single tower. In addition, the studies did notestimate how many carcasses were missed as a result of car-cass removal (scavenging), searcher efficiency, partial-sea-son searches, or unsearchable areas. If these factors wereincluded, the numbers of fatalities reported would likelyhave been at least two to five times greater.

Starting in 2003, we initiated studies to quantify fatalitiesand, more important, to identify ways to reduce fatalities(Gehring et al. 2009, in press). The first of those studies wasconducted between 2003 and 2005 at 24 towers in Michi-gan. The study examined the roles of tower height (380–485feet vs. >1,000 feet), guy wires (guyed vs. free standing), andFAA lighting types (flashing red strobes vs. white strobes vs.steady burning red lights) as they relate to numbers of fatal-ities at towers. Four towers studied were greater than 1,000

feet in height; they were television towers. Shorter towerswere owned and operated by the Michigan Public SafetyCommunication System (MPSCS, Michigan State Police).For all of these towers, scavenging rates, searcher efficiency,and searchable area were estimated, thereby permitting es-timates of how many birds were actually killed rather thanhow many carcasses were recovered.

Gehring et al. (2009, in press) made three importantfindings. First, at towers with guy wires and the same typesof FAA lighting, the fatality rate was about four timesgreater at towers >1000 feet compared to towers of 380–485 feet. Second, there were 16 times more fatalities atguyed towers vs. unguyed towers of the same height (380–485 feet) and with the same FAA lighting. Third, 50–71%fewer fatalities occurred at guyed towers 380–485 feet inheight after standard, steady burning red FAA lights wereextinguished and only flashing lights (red strobe, whitestrobe, or red incandescent) remained .

While conducting the MPSCS studies, we started to realizethat publicly owned towers in the mid-height range (300–500+ feet), such as those used for the newer 800 megahertzsystems, are being developed rapidly across the U.S. The state

Northern Parula. Cape May, New Jersey; September2010. Photo by © Michael O’Brien.

Ovenbird. Cape May, New Jersey; August 2010.Photo by © Michael O’Brien.

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of Michigan has erected about 170 of these towers, whereasneighboring Ohio and Illinois have erected about 205 andabout 187 of these towers, respectively. Many of these towersare guyed and have standard FAA lighting (red flashing andred steady burning) that is known to attract birds and causecollisions. We estimate that public safety collectively kill tensof thousands of birds each year. Although these towers havebeen licensed by the FCC, and presumably by state govern-ments, it appears that state and federal wildlife agencies havehad little or no oversight in the permitting process. To ourknowledge, these agencies have not mandated post-construc-tion studies to determine how many birds are killed; neitherhas the permitting process allowed for mitigation measures toreduce fatality rates.

The overall number of fatalities occurring at communi-

The Red-eyed Vireo is estimated by Partners in Flight to be themost abundant neotropical migrant in North America. As allbirders know, these common vireos spend much of the daytimehours in summer in leafy forests across a wide swath of NorthAmerica (left). Even though they can seem rather dopey andinert on the breeding grounds, Red-eyed Vireos are powerfullong-distance migrants (right). Unfortunately, many perishduring collisions—mainly at night—with communicationstowers, wind turbines, and buildings (right).

Clockwise from left:

Litchfield County, Connecticut; June 2006. Photo by © Brian E. Small.

Cape May, New Jersey; September 2008. Photo by © Michael O’Brien.

Pennsylvania; date unknown. Photo courtesy of © Paul Kerlinger.

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cation towers is not known, although some re-searchers have calculated various estimates. The firstestimate of 1.25 million birds per year was publishedby the U.S. Fish and Wildlife Service (Banks 1979).In the 1990s and early 2000s, estimates were be-tween four and 50 million (Manville 2001, Manville2005). These estimates were based on a limitednumber of studies, which not include adjustmentsfor scavenging, searcher efficiency, unsearchablearea, or seasonal bias. Until our studies in Michigan,quantitative estimates of how many birds are actuallykilled at communication towers of differing struc-tures were not available. Such estimates must in-clude the above adjustments to determine howmany carcasses were not found. Also, without stud-ies from other parts of the U.S. similar to those ofGehring et al. (2009, in press), overall fatality esti-mates will remain elusive.

Wind TurbinesAs of early 2010, there were about 31,000 utility-scale wind turbines operating in the U.S.—and oth-ers in Canada. Some of these turbines date to the late1980s, but most were built after 2000. Since about

Top: Although this article focuses on communicationstowers and wind turbines, it is important to bear inmind that collisions with buildings—especially thosewith extensive windows—are an additional hazard tomigrating birds, like this Dark-eyed Junco. Stillwater,Oklahoma; March 2010. Photo by © Tim O’Connell.

Middle: Until recently, studies of avian collisions withstructures have been highly anecdotal. There is also anunderstandable emotional response to seeing a birdkilled by a building or other structure—for example,this Sprague’s Pipit on the Oklahoma State UniversityCampus. But what are the actual effects, if any, on birdpopulations? Stillwater, Oklahoma; March 2010.Photo by © Vince Cavalieri.

Bottom: Until recently, estimates of bird collisions withtowers, turbines, and buildings have been based on countsof carcasses found beneath such structures. Such studies,however, have been seriously impaired by biases relatedto searcher efficiency and scavenger removal. Upper:Yellow-billed Cuckoo. Lower: Black-and-white Warbler.Stillwater, Oklahoma; June 2010. Photo by © Tim O’Connell.

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2005, new utility-scale turbines have ranged between about350 and 420 feet in height. They consist of a cylindricalsteel tower about 265 feet in height, on which the turbineis mounted inside a box-like or cylindrical structure calleda nacelle. The rotor consists of three blades, which aremounted on the nacelle; the rotor diameter is currentlyabout 265 feet, although there are plans for rotors 328 feetin diameter or longer. The blades range from about 115 tonearly 164 feet in length. Mounted on the nacelle are FAAobstruction lights, usually red-flashing strobe-like lights.

To determine collision fatality impacts to birds and batsat wind turbines, post-construction fatality studies havebeen conducted in at least 21 states. To date, impact studieshave been or are being conducted at about 50 wind plantsaround the U.S. and Canada. We now know more aboutthe impacts of wind turbines (~1% of our electricity) towildlife than we do about impacts from coal, oil, or naturalgas–fired electric generation (>70% of our electricity).

Unlike most fatality studies conducted at communicationtowers, studies at wind turbines have estimated total fatal-ities by adjusting for scavenging, searcher efficiency, and thesearchable area beneath turbines. In other words, the actualfatality rate per turbine or per megawatt at a wind plant iscalculated upward from the number of carcasses found.Most studies have determined that there were 2–5 times asmany birds killed as actually found, so calculations using

scavenging, searcher efficiency, and search-area adjustmentsprovide a relatively accurate estimate of overall fatalities.

Wind turbines were first noted to kill Golden Eagles andother raptors in the Altamont Pass of California. Night-mi-grant songbirds account for 70–80% fatalities in the easternU.S. Common birds, such as Red-eyed Vireos, kinglets, andwarblers, are killed most often. In the western U.S., theHorned Lark appears to be the bird species most affected.No federally endangered or threatened bird species hasbeen documented to be killed to date. As with communi-cation towers, fewer night migrants are killed at wind tur-bines in the western U.S. compared to the east. Raptors,waterfowl, shorebirds, gamebirds, and some others arekilled by turbines, but in much smaller numbers thansongbirds. Overall fatality rates at wind turbines range fromless than 1 bird per turbine per year at some western sitesto perhaps 6–7 birds per turbine per year at some easternsites. The average across the U.S. is around 3–4 birds perturbine. This translates to roughly one bird killed per 80–150 homes for which electricity is generated.

Because night migrants account for a large proportion offatalities at most eastern and midwestern wind plants, re-cent interest has focused on FAA lighting. Kerlinger et al.(2010) reviewed fatality studies at more than 30 wind-power facilities in the U.S. and Canada to determinewhether turbines with FAA red flashing strobe-like lights

Left: Although songbirds repre-sent the bulk of collisions withstructures, other bird speciesare affected. This NorthernBobwhite was found beneatha wind turbine. WoodwardCounty, Oklahoma; June 2005.Photo by © Martin Piorkowski.

Right: Wind turbines maypose a significant threatto bats, like this MexicanFree-tailed Bat. Preliminaryresearch suggests that batfatalities during fall migration(August–early September) maybe greatly reduced by shuttingdown turbines on nights whenwinds are less than 10–12miles per hour. WoodwardCounty, Oklahoma; June 2005.Photo by © Martin Piorkowski.

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are responsible for killing more night migrants than tur-bines without such lights. Relying on more than 25,000 in-dividual turbine searches conducted only during themigration seasons, they found no significant difference infatality rates between lit and unlit turbines. This simplefact, along with the absence of guy wires on wind turbines,explains why the numbers of fatalities at wind turbines isless than at guyed communication towers equipped withboth red flashing and red steady burning lights.

As of early 2010, we estimated that approximately100,000 birds are killed by wind turbines each year in theU.S., based on an average of about 3+ birds per turbine peryear times 30,000 turbines. This number of fatalities doesnot appear to be causing significant impacts to populationsof the species involved, although as more turbines areerected, cumulative impacts must be considered. If thenumber of birds estimated to be killed by communicationtowers (4–50 million) is correct, towers cumulatively maykill 40–500 times more birds than do wind turbines cur-rently operating in the U.S.

The FutureAlthough fatalities at communication towers are likely to in-crease in the coming years as more towers are built, measuresmay be taken to reduce fatalities. Most important, replacingaging existing towers with shorter, unguyed towers could re-duce fatalities by at least an order of magnitude. Lightingchanges are the easiest and most cost-effective means of re-ducing fatalities. If, as Gehring et al. (2009) recommended,steady burning lights are eliminated or replaced with flashinglights, fatalities at guyed towers may be reduced by up to70%. The feasibility of removing steady burning lights or re-placing those lights with flashing lights is now being consid-ered by the FAA. This simple retrofit would not only savemore than one million birds per year, but might also save en-ergy and reduce operational costs.

Wind turbine mortality will likely increase as more windturbines are erected. However, the types of species affectedmay, to some degree, be controlled by locating turbinesaway from areas used by endangered, threatened, or rarespecies. Perhaps more important from the cumulative fatal-ity perspective, the height of turbines should be limited toless than about 500 feet. That would insure that fatalityrates of night-migrating birds—particularly songbirds—would not increase.

Research conducted in the past decade at communica-

tion towers and wind turbines has greatly advanced ourknowledge and provided robust estimates of the numberand types of species impacted. While fatalities at wind tur-bines may be difficult to reduce via mitigation, it is impor-tant to remember that fatalities at these structures occur atrelatively low levels. The new research, especially for com-munication towers, is encouraging because it demonstratesthat there are relatively easy and cost-effective ways to re-duce fatalities at these structures.

AcknowledgmentsMany government agencies and wind power companies havesponsored the research described here. Full details are pro-vided online: aba.org/birding/v43n1p5w1.pdf

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